Dynamic expansion in recently introduced populations of fire ant parasitoids (Diptera: Phoridae)

Combating invasive species requires a detailed, mechanistic understanding of the manner and speed with which organisms expand their ranges. Biological control efforts provide an opportunity to study the process of species invasions and range expansions under known initial conditions. This study examines the rate, pattern and mechanisms of spread for two populations of the biological control agent Pseudacteon tricuspis, phorid-fly parasitoids of imported fire ants. We employ a trap-based survey method that detects phorid flies in low-density populations, and provides data on abundance. This technique allows us to differentiate between continuous population spread and effective long-distance dispersal and to examine density gradients of phorid flies across the expanding population front. We find that occupied sites in front of the leading edge of continuous populations were common; forming small populations we refer to as satellite populations. Satellite populations are tens of kilometers from the nearest possible source. Wind governs the dynamics of spread in these two central Texas populations. Population edges expanding with the wind exhibited a higher frequency of effective long-distance dispersal than did populations expanding into the wind. This enhanced effective long-distance dispersal rate translated into a five times faster rate of spread for population edges traveling with the wind. This planned invasion shares many characteristics in common with unplanned species invasions including: protracted establishment phase during which densities were below detection thresholds, and slow initial spread immediately after establishment followed by rapid, accelerating spread rates as population sizes grew.     

动物入侵的行为机制

行为特征可在外来动物建立种群和扩张过程中发挥重要作用,因此,要正确理解动物入侵,常常需要仔细研究其行为机制。20世纪80年代以来,随着动物入侵规模在世界各地的迅速加剧,有关其行为机制的研究也受到了广泛关注。最近一些研究表明,一些入侵动物种内攻击和觅食等行为具有可塑性,因此它们能够灵活应对多变的环境条件,这对于种群的建立和维持至关重要;入侵动物与土著物种发生行为互作时,往往占据优势,从而取代土著物种,并有助于其地域扩张;入侵动物长距离扩散可以提高其地域扩张速度,许多行为可与扩散行为结合进一步促进扩张。今后需要加强对入侵动物的行为分析,使之全面地融合到生物入侵的研究之中。这不仅可以提高对外来物种入侵的预警和治理能力,而且为探索动物行为的奥秘以及动物间行为互作在物种进化中的意义提供了独特的机会。     

A simulation model of plant invasion: long-distance dispersal determines the pattern of spread

Mechanisms and consequences of biological invasions are a global issue. Yet, one of the key aspects, the initial phase of invasion, is rarely observed in detail. Data from aerial photographs covering the spread of Heracleum mantegazzianum (Apiaceae, native to Caucasus) on a local scale of hectares in the Czech Republic from the beginning of invasion were used as an input for an individual-based model (IBM), based on small-scale and short-time data. To capture the population development inferred from the photographs, long-distance seed dispersal, changes in landscape structures and suitability of landscape elements to invasion by H. mantegazzianum were implemented in the model. The model was used to address (1) the role of long-distance dispersal in regional invasion dynamics, and (2) the effect of land-use changes on the progress of the invasion. Simulations showed that already small fractions of seed subjected to long-distance dispersal, as determined by systematic comparison of field data and modelling results, had an over-proportional effect on the spread of this species. The effect of land-use changes on the simulated course of invasion depends on the actual level of habitat saturation; it is larger for populations covering a high proportion of available habitat area than for those in the initial phase of invasion. Our results indicate how empirical field data and model outputs can be linked more closely with each other to improve the understanding of invasion dynamics. The multi-level, but nevertheless simple structure of our model suggests that it can be used for studying the spread of similar species invading in comparable landscapes.     

A toad more traveled: the heterogeneous invasion dynamics of cane toads in Australia

To predict the spread of invasive species, we need to understand the mechanisms that underlie their range expansion. Assuming random diffusion through homogeneous environments, invasions are expected to progress at a constant rate. However, environmental heterogeneity is expected to alter diffusion rates, especially by slowing invasions as populations encounter suboptimal environmental conditions. Here, we examine how environmental and landscape factors affect the local invasion speeds of cane toads (Chaunus [Bufo] marinus) in Australia. Using high-resolution cane toad data, we demonstrate heterogeneous regional invasion dynamics that include both decelerating and accelerating range expansions. Toad invasion speed increased in regions characterized by high temperatures, heterogeneous topography, low elevations, dense road networks, and high patch connectivity. Regional increases in the toad invasion rate might be caused by environmental conditions that facilitate toad reproduction and movement, by the evolution of long-distance dispersal ability, or by some combination of these factors. In any case, theoretical predictions that neglect environmental influences on dispersal at multiple spatial scales may prove to be inaccurate. Early predictions of cane toad range expansion rates that assumed constant diffusion across homogeneous landscapes already have been proved wrong. Future attempts to predict range dynamics for invasive species should consider heterogeneity in (1) the environmental factors that determine dispersal rates and (2) the mobility of invasive populations because dispersal-relevant traits can evolve in exotic habitats. As an invasive species spreads, it is likely to encounter conditions that influence dispersal rates via one or both of these mechanisms.     

Social personality polymorphism and the spread of invasive species: a model

Ecological invasions are a major worldwide problem exacting tremendous economic and ecological costs. Efforts to explain variability in invasion speed and impact by searching for combinations of ecological conditions and species traits associated with invasions have met with mixed success. We use a simulation model that integrates insights from life-history theory, animal personalities, network theory, and spatial ecology to derive a new mechanism for explaining variation in animal invasion success. We show that spread occurs most rapidly when (1) a species includes a mix of life-history or personality types that differ in density-dependent performance and dispersal tendencies, (2) the differences between types are of intermediate magnitude, and (3) patch connections are intermediate in number and widely spread. Within-species polymorphism in phenotype (e.g., life-history strategies or personality), a feature not included in previous models, is important for overcoming the fact that different traits are associated with success in different stages of the invasion process. Polymorphism in sociability (a personality type) increases the speed of the invasion front, since asocial individuals colonize empty patches and facilitate the local growth of social types that, in turn, induce faster dispersal by asocials at the invasion edge. The results hold implications for the prediction of invasion impacts and the classification of traits associated with invasiveness.     

Slow spread of the aggressive invader, <Emphasis Type="Italic">Microstegium vimineum</Emphasis> (Japanese stiltgrass)

Microstegium vimineum (Japanese stiltgrass) is a non-native weed whose rapid invasion threatens native diversity and regeneration in forests. Using data from a 4 year experiment tracking new invasions in different habitats, we developed a spatial model of patch growth, using maximum likelihood techniques to estimate dispersal and population growth parameters. The patches expanded surprisingly slowly: in the final year, the majority of new seedlings were still within 1 m of the original patch. The influence of habitat was not as strong as anticipated, although patches created in roadside and wet meadow habitats tended to expand more rapidly and had greater reproductive ratios. The long-term projections of the patch growth model suggest much slower spread than has typically been observed for M. vimineum. The small scale of natural dispersal suggests that human-mediated dispersal, likely influenced by forest road management, is responsible for the rapid spread of this invasive species.     

<Emphasis Type="Italic">PRUNUS</Emphasis>: a spatially explicit demographic model to study plant invasions in stochastic,heterogeneous environments

To model the invasion of Prunus serotina invasion within a real forest landscape we built a spatially explicit, non-linear Markov chain which incorporated a stage-structured population matrix and dispersal functions. Sensitivity analyses were subsequently conducted to identify key processes controlling the spatial spread of the invader, testing the hypothesis that the landscape invasion patterns are driven in the most part by disturbance patterns, local demographical processes controlling propagule pressure, habitat suitability, and long-distance dispersal. When offspring emigration was considered as a density-dependent phenomenon, local demographic factors generated invasion patterns at larger spatial scales through three factors: adult longevity; adult fecundity; and the intensity of self-thinning during stand development. Three other factors acted at the landscape scale: habitat quality, which determined the proportion of the landscape mosaic which was potentially invasible; disturbances, which determined when suitable habitats became temporarily invasible; and the existence of long distance dispersal events, which determined how far from the existing source populations new founder populations could be created. As a flexible “all-in-one” model, PRUNUS offers perspectives for generalization to other plant invasions, and the study of interactions between key processes at multiple spatial scales.     

Modelling the introduction and spread of non‐native species: international trade and climate change drive ragweed invasion

Biological invasions are a major driver of global change, for which models can attribute causes, assess impacts and guide management. However, invasion models typically focus on spread from known introduction points or non‐native distributions and ignore the transport processes by which species arrive. Here, we developed a simulation model to understand and describe plant invasion at a continental scale, integrating repeated transport through trade pathways, unintentional release events and the population dynamics and local anthropogenic dispersal that drive subsequent spread. We used the model to simulate the invasion of Europe by common ragweed (Ambrosia artemisiifolia), a globally invasive plant that causes serious harm as an aeroallergen and crop weed. Simulations starting in 1950 accurately reproduced ragweed's current distribution, including the presence of records in climatically unsuitable areas as a result of repeated introduction. Furthermore, the model outputs were strongly correlated with spatial and temporal patterns of ragweed pollen concentrations, which are fully independent of the calibration data. The model suggests that recent trends for warmer summers and increased volumes of international trade have accelerated the ragweed invasion. For the latter, long distance dispersal because of trade within the invaded continent is highlighted as a key invasion process, in addition to import from the native range. Biosecurity simulations, whereby transport through trade pathways is halted, showed that effective control is only achieved by early action targeting all relevant pathways. We conclude that invasion models would benefit from integrating introduction processes (transport and release) with spread dynamics, to better represent propagule pressure from native sources as well as mechanisms for long‐distance dispersal within invaded continents. Ultimately, such integration may facilitate better prediction of spatial and temporal variation in invasion risk and provide useful guidance for management strategies to reduce the impacts of invasion.     

Aerial Dispersal and Multiple-Scale Spread of Epidemic Disease

Disease spread has traditionally been described as a traveling wave of constant velocity. However, aerially dispersed pathogens capable of long-distance dispersal often have dispersal gradients with extended tails that could result in acceleration of the epidemic front. We evaluated empirical data with a simple model of disease spread that incorporates logistic growth in time with an inverse power function for dispersal. The scale invariance of the power law dispersal function implies its applicability at any spatial scale; indeed, the model successfully described epidemics ranging over six orders of magnitude, from experimental field plots to continental-scale epidemics of both plant and animal diseases. The distance traveled by epidemic fronts approximately doubled per unit time, velocity increased linearly with distance (slope ~½), and the exponent of the inverse power law was approximately 2. We found that it also may be possible to scale epidemics to account for initial outbreak focus size and the frequency of susceptible hosts. These relationships improve understanding of the geographic spread of emerging diseases, and facilitate the development of methods for predicting and preventing epidemics of plants, animals, and humans caused by pathogens that are capable of long-distance dispersal.     

Effects of dispersal mechanisms on spatio-temporal development of epidemics

The nature of pathogen transport mechanisms strongly determines the spatial pattern of disease and, through this, the dynamics and persistence of epidemics in plant populations. Up to recently, the range of possible mechanisms or interactions assumed by epidemic models has been limited: either independent of the location of individuals (mean-field models) or restricted to local contacts (between nearest neighbours or decaying exponentially with distance). Real dispersal processes are likely to lie between these two extremes, and many are well described by long-tailed contact kernels such as power laws. We investigate the effect of different spatial dispersal mechanisms on the spatio-temporal spread of disease epidemics by simulating a stochastic Susceptible-infective model motivated by previous data analyses. Both long-term stationary behaviour (in the presence of a control or recovery process) and transient behaviour (which varies widely within and between epidemics) are examined. We demonstrate the relationship between epidemic size and disease pattern (characterized by spatial autocorrelation), and its dependence on dispersal and infectivity parameters. Special attention is given to boundary effects, which can decrease disease levels significantly relative to standard, periodic geometries in cases of long-distance dispersal. We propose and test a definition of transient duration which captures the dependence of transients on dispersal mechanisms. We outline an analytical approach that represents the behaviour of the spatially-explicit model, and use it to prove that the epidemic size is predicted exactly by the mean-field model (in the limit of an infinite system) when dispersal is sufficiently long ranged (i.e. when the power-law exponent a相似文献   

Human-mediated dispersal of seeds over long distances

Human activities have fundamental impacts on the distribution of species through altered land use, but also directly by dispersal of propagules. Rare long-distance dispersal events have a disproportionate importance for the spread of species including invasions. While it is widely accepted that humans may act as vectors of long-distance dispersal, there are few studies that quantify this process. We studied in detail a mechanism of human-mediated dispersal (HMD). For two plant species we measured, over a wide range of distances, how many seeds are carried by humans on shoes. While over half of the seeds fell off within 5m, seeds were regularly still attached to shoes after 5 km. Semi-mechanistic models were fitted, and these suggested that long-distance dispersal on shoes is facilitated by decreasing seed detachment probability with distance. Mechanistic modelling showed that the primary vector, wind, was less important as an agent of long-distance dispersal, dispersing seeds less than 250 m. Full dispersal kernels were derived by combining the models for primary dispersal by wind and secondary dispersal by humans. These suggest that walking humans can disperse seeds to very long distances, up to at least 10 km, and provide some of the first quantified dispersal kernels for HMD.     

Biological and environmental effects on fine-scale seed dispersal of an invasive tree in a secondary subtropical forest

Dispersal abilities of invading species emerge from the interaction between the species and some features of the target community. Ligustrum lucidum is a tree species invading different ecosystems. Major spatial patterns of Ligustrum invasions and their ecological consequences have been analyzed, but no study addressed the dispersal process at a fine scale, assessing the effects of different biological and environmental factors. Ligustrum lucidum is an ornithochoric species. The structure of the environment determines bird movements and thus affects seed dispersal. We used inverse modeling to analyze bird-mediated dispersal of L. lucidum seeds in a secondary Yungas forest and surrounding crop-fields. We assessed the effects of egestion mode (regurgitation and defecation) and tree density (as an environment character) on seed dispersal. Seed dispersal presented different spatial patterns depending on the egestion mode. Tree density was positively associated with the number of regurgitated dispersed seeds and negatively associated with the number of defecated dispersed seeds. In both cases, dispersal distance increased in open areas, but absence of perches inhibited seed arrival. Thus, spread of L. lucidum is facilitated in open areas with some trees; inside the native forest, short distance dispersal facilitates the gradual invasion by this exotic species. Our results suggest that processes like crop abandonment and forest succession, which are active in subtropical montane systems, may facilitate L. lucidum invasion. Our seed dispersal models should be combined with actual distribution maps of L. lucidum to identify areas vulnerable to new invasions.     

Plant Invasion Patches – Reconstructing Pattern and Process by Means of Herb-chronology

An understanding of the patterns of spread of invasive plant species requires analysis of the major dispersal mechanisms and of the patch structure of suitable habitats, both of which may be scale-dependent. On a larger scale, information from herbarium or literature records has proved useful for the reconstruction of past spread of invasive plants. The objective of this study is to investigate population development of invasive forbs at the scale of a site or stand (the population scale) by using herb-chronology. The feasibility of this approach has been largely disregarded until now because of the perceived difficulties in determining the age of perennial herbs. However, recent findings suggest that most of the dicotyledonous perennial herbs in the seasonal climates develop annual rings in the roots or subterraneous stems and thus demonstrate a high potential of the method in studies on plant invasions that went almost unnoticed. The spatial position and age (by means of analysis of annual rings) of individual plants were determined in invasion patches of five species of perennial forbs in Germany and in the USA. The data thus obtained revealed different spatio-temporal patterns of population development that are consistent with distinct models of (local) plant spread, including diffuse invasion and front-like invasion patterns, and thus suggest different processes at work in the course of invasion. The results suggest that analysis of spatial age structures is useful (i) to estimate rates of patch expansion, (ii) to distinguish between dispersal- and microsite-limited population development, (iii) to evaluate how different site conditions affect population development, and (iv) to help understand metapopulation dynamics.     

Saddle-Point Approximations,Integrodifference Equations,and Invasions

Invasion, the growth in numbers and spatial spread of a population over time, is a fundamental process in ecology. Governments and businesses expend vast sums to prevent and control invasions of pests and pestilences and to promote invasions of endangered species and biological control agents. Many mathematical models of biological invasions use nonlinear integrodifference equations to describe the growth and dispersal processes and to predict the speed of invasion fronts. Linear models have received less attention, perhaps because they are difficult to simulate for large times. In this paper, we use the saddle-point method, alias the method of steepest descent, to derive asymptotic approximations for the solutions of linear integrodifference equations. We work through five examples, for Gaussian, Laplace, and uniform dispersal kernels in one dimension and for asymmetric Gaussian and radially symmetric Laplace kernels in two dimensions. Our approximations are extremely close to the exact solutions, even for intermediate times. We also employ an empirical saddle-point approximation to predict densities using dispersal data. We use our approximations to examine the effects of censored dispersal data on estimates of invasion speed and population density.     

Offense and defense in landscape-level invasion control

Biological invasions are multi-stage processes comprising chance demographic events, species interactions, and dispersal. Despite this complexity, simple models can increase understanding of the invasion process. We model the spread of aquatic invasive species through a network of lakes to evaluate the effectiveness of two intervention strategies. The first, which we call offense, contains the invader at sources; the second, which we call defense, protects uninvaded destinations. Deterministic models reveal the effects of these intervention strategies on spread rates. Practical applications involve finite collections of uninvaded lakes, however, and we therefore also present a stochastic model to describe how these strategies affect expected times to important invasion milestones. When the goal is to reduce overall spread rates, both approaches agree that offense is better early in invasions, but that defense is better after 1/2 the lakes are invaded. When the goal is to protect areas of high conservation value, however, defensive site protection always provides lower per site introduction rates. Although we focus on lakes, our results are quite general, and could be applied to any discrete habitat patches including, for example, fragmented terrestrial habitats.     

Looking at both sides of the invasion: patterns of colonization in the violet tunicate Botrylloides violaceus

Understanding the ecological and evolutionary forces that shape the genetic structure of invasive populations and facilitate their expansion across a large spectrum of environments is critical for the prediction of spread and management of ongoing invasions. Here, we study the dynamics of postestablishment colonization in the colonial ascidian Botrylloides violaceus, a notorious marine invader. After its initial introduction from the Northwest Pacific, B. violaceus spread rapidly along the Pacific and Atlantic coasts of North America, impacting both aquaculture facilities and natural ecosystems. We compare genetic diversity and patterns of gene flow among 25 populations (N=679) from the West and East coasts, and evaluate the contribution of sexual vs. asexual reproduction to this species' invasion success using data from the mitochondrial cytochrome c oxidase subunit I (COI) gene and 13 nuclear polymorphic microsatellite loci. Our results reveal contrasting patterns of spread in the coastal waters of North America. While the West coast was colonized by noncontiguous (long-distance) dispersal, the East coast invasion appears to have occurred through contiguous (stepping-stone) spread. Molecular data further indicate that although dispersal in colonial ascidians is predominantly achieved through sexually produced propagules, aquaculture practices such as high-pressure washing can facilitate fragmentation and potentially exacerbate infestations and spread via asexual propagules. The results presented here suggest that caution should be used against the general assumption that all invasions, even within a single species, exhibit similar patterns of colonization, as highly contrasting dynamics may transpire in different invaded ranges.     

Resource-dependent dispersal and the speed of biological invasions

Many mobile organisms exhibit resource-dependent movement in which movement rates adjust to changes in local resource densities through changes in either the probability of moving or the distance moved. Such changes may have important consequences for invasions because reductions in resources behind an invasion front may cause higher dispersal while simultaneously reducing population growth behind the front and thus lowering the number of dispersers. Intuiting how the interplay between population growth and dispersal affects invasions is difficult without mathematical models, yet most models assume dispersal rates are constant. Here we present spatial-spread models that allow for consumer-resource interactions and resource-dependent dispersal. Our results show that when resources affect the probability of dispersal, then the invasion dynamics are no different than if resources did not affect dispersal. When resources instead affect the distance dispersed, however, the invasion dynamics are strongly affected by the strength of the consumer-resource interaction, and population cycles behind the wave front lead to fluctuating rates of spread. Our results suggest that for actively dispersing invaders, invasion dynamics can be determined by species interactions. More practically, our work suggests that reducing invader densities behind the front may be a useful method of slowing an invader's rate of spread.     

Interpretation of gypsy moth frontal advance using meteorology in a conditional algorithm

The gypsy moth, Lymantria dispar, is a non-native species that continues to invade areas in North America. It spreads generally through stratified dispersal where local growth and diffusive spread are coupled with long-distance jumps ahead of the leading edge. Long-distance jumps due to anthropogenic movement of life stages is a well-documented spread mechanism. Another mechanism is the atmospheric transport of early instars and adult males, believed to occur over short distances. However, empirical gypsy moth population data continue to support the possibility of alternative methods of long-range dispersal. Such dispersal events seemed to have occurred in the mid- to late-1990s with spread across Lake Michigan to Wisconsin. Such dispersal would be against the prevailing wind flow for the area and would have crossed a significant physical barrier (Lake Michigan). The climatology of the region shows that vigorous cyclones can result in strong easterly winds in the area at the time when early instars are present. It is hypothesized that these storms would enable individuals to be blown across the Lake and explain the appearance of new population centers observed at several locations on the western shore of Lake Michigan nearly simultaneously. A synoptic climatology model coupled with population dynamics data from the area was parameterized to show an association between transport events and population spread from 1996 to 2007. This work highlights the importance of atmospheric transport events relative to the invasion dynamics of the gypsy moth, and serves as a model for understanding this mechanism of spread in other related biological invasions.     

Modelling biological invasions over homogeneous and inhomogeneous landscapes using level set methods

The establishment, spread and subsequent degradation of existing environments by invasive species is a worldwide problem affecting native and agricultural ecosystems. The phenomenal cost to governments as a result of research and eradication or control drives the need to understand invasion characteristics. In this paper we develop a method for modelling the boundary of an invasion over time with model inputs being the initial distribution of the invasion and the speed at which the invasion front moves over time. This speed function can depend on the topography of the ground cover and we consider examples of homogeneous and inhomogeneous spread. The possibility of a long-distance dispersal event occurring is also considered. In particular, examples of the spread of emergent weeds and weeds which favour creeks and river beds in New Zealand are presented.     

Invasion speeds in fluctuating environments

Biological invasions are increasingly frequent and have dramatic ecological and economic consequences. A key to coping with invasive species is our ability to predict their rates of spread. Traditional models of biological invasions assume that the environment is temporally constant. We examine the consequences for invasion speed of periodic and stochastic fluctuations in population growth rates and in dispersal distributions.